1. Field of the Invention
The present invention relates to a linear actuator it is particularly concerned with a linear actuator for use in a disk unit, but the present invention is not restricted to disk units, and the linear actuator of the present invention may be used for other purposes, such as in a graphical plotter, printer, or semiconductor mounting device.
2. Summary of the Prior Art
In a disk drive, a disk (optical disk, magnetic disk) is supported for rotation by a suitable support, and the unit also has a head for reading or writing information to the disk. The head is movable linearly so that the combination of disk rotation and head movement permits the whole of the disk to be scanned by the head. It is therefore necessary to provide a linear actuator for moving the head towards or away from the axis of rotation of the disk.
The standard method of forming a linear actuator for this purpose makes use of voice coil motors. In such an arrangement, one or more coils are used to generate a magnetic field, and changes in that magnetic field cause a movable member to move, the movable member being connected to the head. Normally, such an arrangement is also provided with guide rails forming at least one linear guide for guiding the relative movement of the movable member, and hence the head, relative to the rest of the disk unit.
There is a desire to reduce the size of disk units, and the size of the linear actuator is increasingly significant in determining the overall size of the disk unit. With a linear actuator based on voice coil motors, as discussed above, the driving force per unit volume of the coils is small, and hence relatively large coils are needed. Furthermore, at least that part or parts of the movable member which interact with the magnetic fields of the coils needs to be heavy, so that high-speed movement is difficult. The need for large coils is particularly serious when attempts are made to reduce the size of the disk drive, since, in practice, a linear actuator involving voice coil motors has minimum thickness of about 15 to 16 mm, if suitable drive forces are to be generated, whereas it is desirable to reduce the thickness of the disk unit to e.g. 5 to 6 mm.
Therefore, consideration has been given to the use of linear pulse motors. A standard type of linear pulse motor, known as the "hybrid type", involves two members each with at least one set of teeth with a predetermined tooth spacing pitch and each set of teeth being magnetically associated with a drive coil for generating magnetic fields. The two members are arranged so that the teeth of one member oppose the other, and by controlling and varying the drive current to the coils, and hence the magnetic polarity of the teeth, the two members may be caused to move. In such an arrangement, the parts of the members which form the teeth are normally iron. It is relatively easy to make such a hybrid type linear pulse motor relatively thin, but the arrangement has a high inductance, giving a slow response time, and the fact that both the fixed member and the movable member have teeth makes the linear actuator difficult to construct.
Therefore, proposals have been made to make use of a permanent magnet (PM) type linear pulse motor, in which one of the members carries at least one track of permanent magnets, with those magnets being arranged so that adjacent magnets have opposite magnetic polarity.
JP-A-1-174262 (corresponding to U.S. Pat. No. 4,945,268) proposed a linear actuator for use in e.g. a disk unit, in which one member had a track of permanent magnets described above, and there was then a drive part having pairs of sets of teeth with the track of permanent magnets being interposed between the sets of teeth.
This idea was also considered in JP-A-2-246761 (although there was no disclosure in that document of application to a disk unit) in which there were two tracks of permanent magnets, and first and second drive parts. One drive part was arranged so that the drive coils thereof were on one side of the member carrying the tracks, and those drive coils were connected to two pairs of sets of teeth with one track being interposed between those sets of teeth. The drive coils of the other drive part were arranged on the opposite side of the member carrying the track of permanent magnets, again with those drive coils being magnetically connected to pairs of sets of teeth the second track then being interposed between those sets of teeth of the other drive part.
In JP-A-2-246761, the two drive parts were arranged so that they were aligned in a direction perpendicular to the direction of extension of the tracks of permanent magnets, and the magnetic poles of the tracks of permanent magnets were similarly aligned. It was disclosed, however, that the direction of the sets of teeth could be skewed relative to the direction of extension of the tracks of permanent magnets.